Direct in vivo measurement of flow-dependent nitric oxide production in mesenteric resistance arteries

Purpose: To determine whether the concentration of nitric oxide (NO) at the arterial wall is increased subsequent to the abrupt elevation of blood now in resistance arteries. Methods: Eight dogs underwent laparotomy with anesthesia, and their small bowels were exteriorized. NO concentration was measured with NO-specific electrodes (200-mu-tip diameter) at the outer wall of the mesenteric arteries. Flow was increased by occlusion of the adjacent mesenteric arteries. In four animals, flow and NO concentration were measured after the administration of N omega-nitro-L-arginine-methyl ester (L-NAME) to inhibit NO production. Results: As arterial flow was increased from a baseline of 5.4 +/- 1.3 ml/min to 10.9 +/- 1.8 ml/min (p = 0.001), NO electrode current was elevated in every animal. With repetition of the flow stimulus, the response tended to be attenuated. In the first experimental trial, NO electrode current measured at the arterial wall increased from 2.86 +/- 0.56 to 3.00 +/- 0.60 nA (p = 0.02). L-NAME (10 mg/kg intravenous) effectively inhibited NO synthase as indicated by the elevation of mean arterial pressure (11 +/- 1.7 mm Hg; p = 0.04). After administration of L-NAME, NO electrode current measured at the outer arterial wall fell 0.23 +/- 0.05 nA (p = 0.02). Conclusions: The data indicate that a doubling of blood now in the canine mesenteric resistance arteries is associated with an increase in NO concentration of at least 100 nm at the outer arterial wall. This association is probably a substantial underestimation of the actual concentration because of the geometry of the electrode tip. To our knowledge, ours is the first report of direct in vivo measurement of now-dependent NO release in resistance arteries.